This is a continuation of a study to determine the amino acid requirements of the premature infant. This includes 1) investigation of the possibility that certain amino acids which are not essential for the mature individual are dietary essentials because of immaturity of a number of enzyme systems, and 2) the determination of quantitative needs which are presumably great because of the rapid growth rate. A diet containing 18 L-amino acids as the nitrogen moiety is used, and the criteria of adequacy include weight gain, nitrogen retention and the plasma aminogram. The purpose of the study is to determine which amino acids are required and in what amounts. It is important to have precise knowlede of the amino acid requirements of the premature infant because of their limited capacity to handle quantities of food in the gastrointestinal tract and their deficiences in metabolizing and excreting any excesses.